I am trying to eliminate odors inside HVAC by cooking the mold that is presumably causing them, as suggested in https://mechanics.stackexchange.com/a/146/15334 (which got 39 upvotes)

However, I don't understand what the different knobs actually do internally.

One knob controls the "temperature". I set that to max, obviously.

The other knob controls how quickly air flows into the cabin. Should I set that to max (which heats the cabin the fastest), or should I set it to min, so that the hot air stays in the HVAC system instead of heating the cabin?

(If I knew how it all works internally, and where the mold likely is, I'd figure it out, but I don't)

Edit: found this illustration on agcoauto.com, which basically confirms what sweber wrote:

enter image description here

2 Answers 2


The reason for the odor is that water condenses inside the ducts when the AC cools the air. Though there are drains, there will still be enough moisture in the system to allow mold to grow.

The heating is usually placed after the AC. This is useful for screen defogging: The air is first dehumidified by the AC and then heated, which gives warm, extremely dry air. The other way, dehumidifying would not work.

So I think you can't cook your mold, because it's sitting in the AC system, which you can't heat.

Furthermore, mold dies when the temperature rises above 70°C for a longer period, may be one or two hours. This heavily depends on the type of the mold. Coolant of a warm motor typically has about 80-90°C, but it's possible that it has already cooled down a little when it flows into the heating part of the HVAC system. And the air may also not be fully heated to the temperature of the coolant. So, it's possible that the hot air is not hot enough to kill the mold. However, the air would be hottest for lowest fan speed.

So, contrary to the answer you linked, I don't see how heating can help to dry the entire system. It only helps to dry the ducts between heater and vents, but not to dry the AC.

The other answer is better: Run the system (also at high fan speed) without AC for some minutes to get as much humidity as possible out of the system.

Also keep in mind: Once there is dead mold, this is perfect food for the next mold...

  • I don't see how heating can help to dry the entire system But if the fans are off, would the hot air not go everywhere, including back to the AC? (I still don't have a very good idea of how it's all connected. Does the temperature knob redirect the air so it doesn't even go through AC?)
    – MWB
    Mar 2, 2016 at 16:12
  • Hot air alone may evaporate some of the moisture. But then, it has to leave the system. Otherwise, the moisture will condense again when the air cools down.
    – sweber
    Mar 2, 2016 at 16:15
  • If you can go without the ac for a while you should be able to dry it out. When idling, turning off the fan and setting full heat should allow some warm air to diffuse around the system. Maximum airflow on a warm dry day will evaporate water in the ac core.
    – Chris H
    Mar 3, 2016 at 9:29
  • @ChrisH I've gone without AC for months (the summer is over), and I still smell something often, but I don't really use the heater much either.
    – MWB
    Mar 3, 2016 at 20:16
  • @MaxB, you probably don't have the fan on high much either.
    – Chris H
    Mar 4, 2016 at 9:46

Use a spray can of deodorizer like this or similar, found at most auto parts stores. It sprays directly into the air inlet below the windshield on the exterior of the vehicle while the fan is on to pull it through the system and out the vents. As sweber stated there's no way the cars heater is going to fix this problem.

I stop the nasty smell from starting by turning off my A/C compressor and leaving the fan on pulling outside air for the last few minutes or so before reaching my destination. This causes the hot outside air to dry out the evaporator so the moisture doesn't sit in the system inside the hot car after it's off. The last few minutes may get a bit warm, but my car never smells when I get back in it.

  • This goes into the drain at the bottom, right? I'm not really set up to do that myself. I wonder if A/C evaporator coil cleaning is a standard service that auto mechanics provide.
    – MWB
    Mar 2, 2016 at 18:25
  • Nope, this gets sprayed into the air intake outside of the car at the bottom of the windshield, and it you have access to it you can also spray some into the interior intake (when in recirc mode). The can itself will have more detailed instructions, but this is something you can easily do yourself.
    – Nick G
    Mar 2, 2016 at 18:55

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